Duplicate links attribute check should not count unused optional dependencies

[toyboot4e]

Problem

cargo build fails if Cargo.toml has two different sys crates that link to the same C library:

[dependencies]
# they both link to `OurLibrary`
sys-crate-A = { version = "..", optional = true }
sys-crate-B = { version = "..", optional = true }

..even if one (or none) of them are used.

cargo build says as this:

the package `sys-crate-B` links to the native library `OurLibrary`, but it conflicts with a previous package which links to `sys-crate-A` as well:

Possible Solution(s)

Unused optional crates should be excluded from the duplicate links attribute check. I guess.

Notes

This repository contains concreate example and full error message. Thank you.

[toyboot4e]

Excuse me.. it was duplicate of #6231 and such.